The three British soldiers killed by an Afghan policeman in a "green on blue" attack on Sunday have been named by the Ministry of Defence.

Warrant Officer Leonard Thomas, of the Royal Corps of Signals, and Guardsmen Craig Roderick and Apete Tuisovurua, both of 1st Battalion the Welsh Guards, were killed when a policeman at a checkpoint in Helmand province opened fire.

The soldiers had been conducting a routine visit to the compound – known as Kamparack Pul – for a meeting with a detachment of the Afghan national civil order police (Ancop). The gunman, an Ancop officer, was arrested at the scene.

The MoD said Thomas, 44, from Ross-on-Wye, Herefordshire, was on his last tour before leaving the military. He served in the Coldstream Guards and Welsh Guards before joining the signals corps. He had served in Northern Ireland and Iraq. His partner, Sergeant Rachel Prosser, is also in the army.

Roderick, 22, from Cardiff, joined the Welsh Guards in September 2009, based in Aldershot. He leaves his parents, Mike and Sadie, sisters Katie and Lucinda Emily, stepbrother Jay, and girlfriend Zoe.

Fiji-born Tuisovurua, 28, joined the regiment, based in Hounslow, west London, in June 2011. He leaves his parents and seven siblings.

The defence secretary, Philip Hammond, said: "I am saddened to hear of this cowardly act which has taken the lives of three very brave British soldiers. They gave their lives protecting Britain's national security; their efforts will not be in vain and we will always remember them."

An investigation has begun into the incident, the third attack on the British by Afghan colleagues this year.